Tutorials for simple objects like Railway wagons etc
Different functions of Blenders UI is mainly split into different sections and tabs, if you stick to just the modelling sections you can avoid the clutter of buttons for texturing and rendering. You can add reference images by pressing (Shift)+(A) and going to image&gt;Reference. 
To extrude simply select your mesh and press (tab) to enter edit mode. Then select something and press (e) to extrude. You can also snap the extrusion to an axis by pressing x, y, or z.

To activate tools for measuring you can follow this simple tutorial
I also found a tutorial that leads you through a step by step process of modelling in Blender for a 3D printing purpose. Just click here.
If you have any specific questions or need help with how to approach modelling a specific object, feel free to reply to this answer and I can give you some advice.